I don't know if this has anything to do with your case, but I had kind-of-a-similar experience with a DL coded AF operated flight.

I could see the seating map on the DL site and it would let me select seats but refused to save it. When asking DL they said they couldn't help me with an AF flight and I'd have to wait 'till the AF check-in. However, when I turned up at the airport we had the exact premium seats (744 upper deck) I selected at the DL site reserved for us, so apparently the selection got through even though the system did not confirm it. Could this be a similar case if you can see the seating chart but not reserve a seat?

Same thing on the way back, except they moved us from middle and window to aisle and window with a middle blocked, which I did not protest too much.